Process for fluid catalytic cracking of hydrocarbon feedstocks with high levels of basic nitrogen

ABSTRACT

A process is described for fluid catalytic cracking of hydrocarbons with high levels of basic nitrogen, where hydrocarbon feedstocks A and B with different levels of basic nitrogen are injected in a segregated fashion, into different risers of a multiple riser FCCU that possesses at least two risers. The injection of the feedstocks is made in such a way that feedstock A, to be injected in the riser with greater volume—main riser  7 —possessing a level of basic nitrogen at least 200 ppm lower than the level of feedstock B to be injected into the riser with lower volume—secondary riser ( 8 ).

This application is a continuation of U.S. application Ser. No.10/689,662, filed Oct. 22, 2003, the entire content of which isincorporated by reference in this application.

FIELD OF THE INVENTION

The present invention relates to a process for fluid catalytic cracking(FCC) of hydrocarbon feedstocks with high levels of basic nitrogen inFCC units (FCCU) provided with multiple conversion zones (risers). Morespecifically, the invention relates to a process for fluid catalyticcracking of hydrocarbon feedstocks with different levels of basicnitrogen, which uses a segregating route of said hydrocarbon feedstocksthat are fed into different risers in the multiple risers FCCU.

BACKGROUND OF THE INVENTION

There have been ever increasing oil discoveries in Brazil that containhigh concentrations of basic nitrogenous compounds, be they aromatic orpolyaromatic, that may or may not be branched, with predominantlyheterocyclic chemical structures, that are concentrated in high boilingpoint hydrocarbons fractions such as Heavy Gas Oil (HGO), AtmosphericResidue (AR), Vacuum Residue (VR), by-products of the distillationprocess, among others, such as Heavy Coker Gas Oil (HCGO), by-productsof delayed coking units, and even deasphalted oil (DESO) a by-product ofasphalt production units.

Basic nitrogenous compounds, when present in fluid catalytic crackingprocessing, feedstocks tend to promote deactivation of the catalyst acidsites and to increase the level of coke deposits on the catalyst, withthe subsequent loss of product conversion and selectivity in theprocess.

Fluid catalytic cracking (FCC) is performed by the contact ofhydrocarbons in a reaction zone with a catalyst made up of fineparticulate matter. Feedstocks that are commonly submitted to FCCprocessing are, usually, petroleum refinery process streams that comefrom longitudinally segmented vacuum towers, called Heavy Vacuum Gas Oil(HVGO), streams coming from delayed coking units, Heavy Coker Gas Oil(HCGO) or, heavier that the former, coming from the bottom ofatmospheric towers, Atmospheric Residue (AR), or even mixtures of thesefeedstocks.

These streams, that typically have a density in the range of 8 to 28°API, must be chemically processed using a process such as the catalyticcracking process, which fundamentally alters its composition, convertingthem into lighter, more valuable hydrocarbon streams.

During the cracking reaction, substantial portions of coke, a by-productof the reaction, are deposited on the catalyst. Coke is a material ofhigh molecular weight, made up of hydrocarbons containing, typically,from between 4 and 9% of its compositional weight in hydrogen. Thecatalyst covered with coke, usually called “spent catalyst” by thespecialists in the field, is continually removed from the reaction zoneand is substituted with catalyst that is essentially free of coke fromthe regeneration zone.

In the regeneration zone and in a regenerator vessel kept at hightemperature, the coke deposited on the surface and on the pores of thecatalyst is burned off. Removing the coke through its combustion allowsfor a recovery of the catalyst activity and frees heat in sufficientamount to fulfill the thermal requirements for catalytic crackingreactions. The fluidization of the catalyst particles by gaseous feedsallows the catalyst to be transported between the reaction zone and theregeneration zone and vice-versa. The catalyst, aside from fulfillingits main function of expediting the catalyzation of chemical reactions,as well as providing a method for transporting heat from the regeneratorto the reaction zone.

The technique contains many descriptions of hydrocarbon crackingprocesses in a fluidized catalyst feed, with catalyst transportedbetween the reaction zone and the regeneration zone, and coke burning inthe regenerator.

In spite of the long-time existence of the FCC process, techniques toimprove the process have continually been sought to increase theproduction of derivatives of greater aggregate value, such as Naphthaand LPG. Generally speaking, it could be said that the main purpose ofFCC processes is to maximize the production of these more valuablederivatives.

This maximization is basically obtained in two ways. First, byincreasing the so-called “conversion” used to reduce the production ofheavy products such as clarified oil and light recycled oil. And second,by reducing the production of coke and combustible gas, in other words,less “selectivity” towards these products.

A lower production of these last two products, besides expediting anincrease in the production of gasoline and LPG, by increasing theprocess selectivity towards these derivatives, provides as a result theadditional benefit of lower air blower and wet gas compressor use(machines with a high deadweight and large power consumption), which inturn usually cause a limitation of the FCCU capacity.

It is well known that an important aspect of the process and the initialcontact of the catalyst with the, feedstock that exerts a decisiveinfluence on the conversion and the selectivity of the process ingenerating noble products. In the FCC process, the feedstock ofpreheated hydrocarbons is injected next to the base of a conversion zoneor riser, where it enters into contact with the flow of the regeneratedcatalyst, from which it receives sufficient heat to vaporize it andsupply the demand of the endothermic reactions that dominate theprocess.

After the riser, (which is an elongated vertical pipe whose dimensions,in industrial units, are around 0.5 to 2.0 m in diameter by 25 to 40 mhigh, and is where chemical reactions occur) the spent catalyst, withcoke still deposited on its surface and pores, is separated from thereaction products and is sent to the regenerator in order to burn offthe coke so as to restore its activity and to generate the heat that,transferred by the catalyst to the riser, will be used by the process.

The conditions existing at point of the feedstock's entry into the riserare determined by how many products are formed in the reaction. In thisarea an initial mixture occurs of the feedstock with the regeneratedcatalyst, which has been heated to the boiling point of its componentsand to vaporization of the greater part of these components. The totalresidence time of the hydrocarbons in the riser is around 2 seconds. Sothat the catalytic cracking reactions may be processed, vaporization ofthe feedstock in the mixing area with the catalyst must occur rapidly,so that the vaporized hydrocarbon molecules may enter into contact withthe catalyst particles—whose size is close to 60 microns—and permeateinto its micro-pores, undergoing the effect provided by its acid sitesin catalytic cracking. If this rapid vaporization is not achieved,thermal cracking will result of the feedstock's liquid fractions.

It is well known that thermal cracking leads to the formation ofby-products such as coke and combustible gas, mainly in residualfeedstock cracking. Coke, in addition to its low commercial value,obstructs the pores of the catalyst. Therefore, thermal cracking in thebed of the riser competes in an undesirable fashion with catalyticcracking, which is the purpose of the process.

Feedstock conversion optimization usually requires maximal removal ofcoke from the catalyst in the regenerator. Combustion of the coke may beobtained by partial combustion or total combustion. In partialcombustion, the gases produced by combustion of the coke are principallymade up of CO₂, CO and H₂O and the percentage of coke in the regeneratedcatalyst is on the order of between 0.1% a 0.2% by weight. In this caseof total combustion (performed in the presence of a great excess ofoxygen), practically all of the CO produced has already reacted and beenconverted to CO₂. The oxidation reaction of CO to CO₂ is stronglyexothermic, so that when this total combustion happens it releases agreat amount of heat, resulting in very elevated regenerationtemperatures. However, total combustion produces catalyst containingless than 0.07% and, preferably, less than 0.05% in weight of coke,making this feature more advantageous than partial combustion, inaddition to precluding the need to use a burdensome boiler to combustthe CO afterwards.

The increase in coke on the spent catalyst results in an increase in thecoke combustion in the regenerator per unit of mass of circulatedcatalyst. Heat is removed from the regenerator in conventional FCC unitsin the combustion gas and mainly along the regenerated hot catalyststream. An increase in the percentage of coke on the spent catalystincreases the temperature of the regenerated catalyst and the differencebetween the temperatures between the regenerator and the reactor.

Meantime, a reduction in the output of regenerated catalyst towards thereactor, (usually called circulation of the catalyst), is necessary inorder to fulfill the thermal demand of the reactor and to maintain thereaction at a constant temperature. However, the lower catalystcirculation rate demanded by the great difference in temperature betweenthe regenerator and the reactor, which results in a decrease in thecatalyst/oil ratio, which in turn lowers the conversion.

So, the circulation of the catalyst in the regenerator towards thereactor is defined by the thermal demand of the riser and by thetemperature established in the regenerator, (a function of theproduction of coke). Since coke that is generated in the riser isaffected by the circulation of the catalyst itself, a conclusion may bedrawn that the catalytic cracking process works under a system ofthermal balance. However, (for the indicated reasons), very elevatedtemperatures are undesirable in the regeneration operation.

Usually, with modern FCC catalysts, the temperatures of the regeneratorand, consequently, that of the regenerated catalyst, are kept below 760°C., preferably under 732° C., since the loss of activity would be verysevere above this number. A desirable operational range is between 685°C. and 710° C. The lower value is dictated, mainly, by the need toguarantee proper combustion of the coke.

With the ever increasing weight of feedstocks processed, there is atrend towards raising the production of coke and the total combustionoperation requires catalyst coolers to be installed in order to keep thetemperature of the regenerator at acceptable limits. The catalystcoolers usually remove heat from the catalyst stream coming from theregenerator, returning to this vessel a substantially cooled catalyststream.

As regards the fluid-dynamic characteristics of the riser, where thecatalytic cracking reactions are processed of the present invention,what is known is that solid catalyst particles are dragged, by thereaction itself, during contact with the feedstock and other vaporizedmaterials.

These types of reactors usually have the shape of a pipe where, in orderto reduce the production of by-products, it is necessary to operatewithin a hydrodynamic stream system, in such a way as to allow thesurface velocity of the gas to be either high or sufficient enough tocause the catalyst to flow in the same direction as the feedstock andthe other vapors there existing. In other words, the liquid andvaporized feedstock drags the catalyst particles with it through theinput passageway in the pipe reactor.

These stream systems are known by technicians in the field as fastfluidized bed, riser systems, or more generically as transport systems,which are the preferred systems when dealing with reaction systems thatrequire continuous flow reactors.

Usually, for any given area in the cross section of a pipe reactor(which is a function of the diameter of the reactor itself), theconcentration of the catalyst, in the fluidized bed of a reactor,decreases with an increase in the surface velocity of the gas. Thegreater the surface velocity of the gas, the greater the height requiredby the reactor to allow a given quantity of the feedstock to be able tocontact the required amount of catalyst. These greater surfacevelocities (of the gas) require a higher L/D (Length/Diameter) ratio, or“aspect ratio” in the reactor. This ratio is the ratio between theheight of the reactor and its diameter.

Additionally, in many cases, it may be desirable to build fluidized bedreactors with large cross section areas so that considerable feedstockoutput can be achieved with a single reactor. However, when the diameterof the fluidized bed is increased, particularly in the transport system,the height of the reactor must be increased as well. This increase inheight is necessary because a certain minimum height is required in thereactor (L/D ratio) in order to achieve a fully developed flow patternthat is closer to the behavior of a continuous flow reactor.

However, fluidized bed reactors with an elevated L/D ratio areexpensive, difficult to build and maintain because they must have verylarge and heavy separating tanks in the top, containing, in theirinterior, equally heavy equipment, that are targeted at capturing andcontrolling the catalyst flow and the products in the reactor.

Finally, FCC Units with multiple risers, may have small diameterfeedstock conversion zones precisely due to having a multiplicity ofrisers and therefore are able to maintain an adequate L/D ratio topromote the necessary fully developed stream systems, with a reasonablereactor height.

The increase in participation of domestic petroleum, originating fromthe oil fields of Campos Basin, on the coast of the State of Rio deJaneiro, presents some technical problems regarding the refining ofhydrocarbon feedstocks derived from these oils, especially when thepresence of basic organic nitrogenous compounds compromises theperformance of the catalysts used in the fluid catalytic cracking (FCC)process, the major supplier of gasoline, diesel and liquefied petroleumgas (LPG) for domestic consumption.

Basic organic nitrogenous compounds, present in petroleum, arepredominantly, made up of the quinoline, benzoquinoline, alkylpyridines,amides, alkyl and hydroquinolines, acridines and phenanthridinesfamilies. Structurally, they are aromatic and polyaromatic heterocycliccompounds, that may or may not be branched, that accumulate on theheaviest fractions of crude oil in separation processes, mentionedabove. Heavy gas-oil originating from Cabiunas petroleum oil may presentabout 1000 parts basic nitrogen per million (ppm).

Any refinery or specialist in the field of hydrocarbon feedstockrefining will recognize the problems arising from the presence of basicnitrogenous compounds in the refinery process, especially in the FCCprocess: Basic nitrogenous compounds are responsible to a large extentfor deactivation of the cracking catalysts, an increase in the level ofcoke, and gum formation in gasoline. In summary, all this represents aloss of capacity in the catalytic cracking unit with consequent greatfinancial damage to the refinery.

In the first attempt to resolve this problem, or at least to minimizeit, several refineries resorted to changing the catalyst used in thecatalytic cracking units, in the search for a catalyst that would bemore resistant to contamination through basic nitrogenous compounds. Asthe catalytic cracking catalyst in use in the great majority ofrefineries is made up of an acidic crystalline aluminosilicate—azeolite, dispersed in a clay matrix, the poisoning of the catalyst bythe basic nitrogenous compounds occurs precisely by the neutralizationof the zeolite's acid centers that are, in the last analysis, the activecenters for cracking the hydrocarbon molecules of the feedstock.

To overcome the deactivation caused by the basic nitrogenous compounds,many manufacturers of catalytic cracking catalysts have offeredcatalysts with a greater number of acid centers to their clients thatcome from a higher percentage of zeolite in the catalyst or by the useof acid matrices. Said resource may work well when the percentage ofbasic nitrogenous compounds in the hydrocarbon feedstock is low, or,when the basic nitrogenous compounds present have a low molecularweight.

It has been verified, in this last instance, that the basic nitrogenouscompounds of low molecular weight only “poison” (in a reversible way),the acid centers of the catalyst and that, after the catalytic crackingcatalyst regeneration stage, the activity of the cracking catalyst isrestored, momentarily, until the catalyst thus regenerated enters againinto contact with the hydrocarbon feedstock containing basic nitrogenouscompounds. However, when basic nitrogenous compounds are made up ofaromatic or polyaromatic compounds that are of a higher molecularweight, as is the case with the basic nitrogenous compounds present, forexample, in CGOs, the deposit of the molecules of these basicnitrogenous compounds onto the surface of the catalyst particles isirreversible because it neutralizes the acid centers and reduces thespecific area of the catalyst, that loses activity and selectivity. Thisis not, therefore, a good solution for fluid catalytic cracking unitsthat process heavy feedstocks of hydrocarbons with these basic nitrogencharacteristics.

Experimental studies performed on a multipurpose pilot FCC Unit of theApplicant, to evaluate the proposal of multiple injections in riserdescribed in the patent CN 1088246 A, of the Petrochemical ResearchInstitute of China Petrochemical Corp., for feedstock rich incontaminants, disclosed that said proposal was very advantageous inindustrial FCC Units that process Coker Gas Oil, having a level ofaromatic, resins, and basic nitrogenous compounds on the order ofbetween 800 and 1000 ppm.

Studies of catalyst sampling in the risers of an industrial FCC Unit,described by Waldir Martignoni et al, in work presented at the EncontroSul Americano de Refino, held in Manaus, AM, Brazil, in April of 2000,proved that the catalyst presents greater activity in first the 15meters of the riser, where most of the conversion occurs in the process.

The injection of feedstocks considered refractory to cracking, as forexample, CGOs, in positions above the traditional nozzle, where most ofthe process conversion has already occurred, increases, significantly,the process conversion of noble products.

In parallel fashion, studies of acid treatments of CGOs, performed atthe work bench level in the laboratories of the Applicant's ResearchCenter, according to concepts contained in patent PI 9803585-1 A, alsobelonging to the Applicant, disclosed that the basic nitrogenouscompounds present in this CGO present a much more marked reactivity thanthe existing compounds in direct distilled heavy gas oil (DDHGO), a factthat would favor a more marked deactivation of the catalyst.

It is also known that Heavy Coker Gas Oil is a refractory feedstock incracking, considering that HCGOs are a fraction derived from a thermalprocess. The Applicant performed runs in a prototype FCC Unit, with thefeedstock output of 200 kg/h, proving a decrease in the processconversion, when the CGO fraction is mixed with the process feedstock,as will be shown in Example 1 of the present report.

In function of the greatest reactivity with the basic nitrogenouscompounds present in HCGOs and of the lower crackability of this type offeedstock, in comparison with vacuum gas oil, if there were thepossibility of adjusting differentiated operational conditions for HCGOs(mainly through the temperature of the reaction), it would be apromising alternative for the optimization of the industrial unit. Thistemperature adjustment would only be possible, if this stream wereprocessed in a secondary riser of a FCC unit with two or more differentrisers.

Another benefit additional to the above achieved by said feedstocksegregation would be that the segregated DDHGO might have thepossibility of entering into contact with a more active catalyst in themain riser, due to the fact that the basic nitrogen present in the mostcontaminated feedstocks would not neutralize the acid sites of thiscatalyst in this riser, because it would be being fed into anothersecondary riser. This effect would be more pronounced than the effectobserved with multiple injection feedstocks in the same riser.

U.S. Pat. No. 6,156,189 describes a type of alternate injection, inrapid feed cycles, made in the risers of pilot FCC Units with one ormore risers, that, similar to the present invention, is presented as analternative to the processing feedstock mixtures with differentproperties, when feedstocks with different properties are injected inthe same riser. It should be emphasized that, industrially speaking,this procedure is an extremely complicated job, due to the fact that thepatent description suggests alternating feeds in intervals or cycles ofbetween 20 seconds and 2 minutes to achieve an increase in conversion.

Different from the State of the Art, the process of the presentinvention involves simultaneous processing, in fluid catalytic crackingunits with multiple risers, of feedstocks containing differentpercentages of contaminants, especially contamination with basicnitrogen, where said feedstocks contaminated with the catalyst damagingbasic nitrogen compounds are segregated into a secondary riser.Additionally, the present invention even includes the use of coolingstreams in the secondary riser, to adjust the catalyst/oil (CTO) ratioin the risers.

The proposal of the present invention also guarantees that the acidsites of the catalyst in the main riser shall remain more active alongthe length of the riser, extending the beneficial effect theoreticallyachieved by U.S. Pat. No. 6,156,189.

Beyond the additional cited benefit, the fact that if segregatedfeedstocks are processed, this will allow for more operationalflexibility, once the reaction temperature may be altered in each riserand the thermal balance modified, increasing the CTO in the risers.

So, despite the proposal literature, the technology for fluid catalyticcracking process still needs to involve simultaneous processing, influid catalytic cracking units, in multiple risers, for feedstockscontaining different percentages of contaminants, especially of basicnitrogen, where the most contaminated feedstocks are segregated intosecondary risers, these risers which are still cooled by cooling(quenching) fluids. A fluid catalytic cracking process that presentssuch features is described and claimed in the present application.

SUMMARY OF THE INVENTION

The present invention relates to process for fluid catalytic cracking ofhydrocarbon feedstocks with high levels of basic nitrogen, in multipleriser FCCUs, operating with feedstocks A and B, where the processincludes the following stages (steps):

-   -   a) place in contact with a zeolite catalyst, in the main riser        of the FCCU, a hydrocarbon feedstock (a) possessing a level of        basic nitrogen at least 200 ppm lower than feedstock (B) that is        being processed in a secondary riser of the same FCCU;    -   b) simultaneously, place in contact with the same zeolite        catalyst as in a), in a secondary riser of the FCCU, a        hydrocarbon feedstock (B) that includes a mixture made up of        between 95 and 40% in volume, of hydrocarbon streams with a        percentage of catalyst damaging basic nitrogen between 1000 and        3500 ppm, and between 5 and 60% in volume, of a cooling fluid        capable of increasing the circulation of this secondary riser        and of cooling the regenerator, in order to adjust thermal        balance of the FCCU and maintain the circulation of the catalyst        in the main riser, at proper levels, so that the catalyst/oil        ratio remains in the range of between 4.5 and 8.5;    -   c) maintain the operation of the FCCU within the conditions of        catalytic cracking.    -   d) recover cracked reaction products with an increase in bottom        conversion, a greater proportion of gasoline and LPG, at the        same time reducing the proportion of coke and combustible gas.

Therefore, the invention provides the possibility of processing,simultaneously, in different risers in the same FCCU with multiplerisers, segregated feedstocks of hydrocarbons, with different levels ofbasic nitrogen.

The invention still provides for the catalytic cracking processing ofsegregated feedstocks of hydrocarbons with different levels of basicnitrogen, which affords an optimization in the conversion selectivityrates of cracking.

The invention still provides for the catalytic cracking processing ofsegregated feedstocks of hydrocarbons with different levels of basicnitrogen where the production of coke and combustible gas is minimized,while the production of gasoline and LPG is maximized, resulting in abetter economy of the FCC process.

The invention also provides for a process that will allow for moreoperational flexibility, once the reaction temperature may be altered ineach riser and the thermal balance modified, increasing the CTO in bothrisers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 attached is a simplified schematic drawing of a FCCU withmultiple risers, useful in the process of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the term “multiple riser” means that a FCC unit used inthe process of the invention has at least two risers, and possibly, asneeded by the cracking process, three risers.

Still according to the invention, feedstock A, to be cracked in the mainriser, may include a mixture of various feedstock streams, in anyproportion, as needed to keep the difference less than at least 200 ppmof basic nitrogen in feedstock A as compared with feedstock B. FeedstockA may be constituted of a pure stream or of a stream made of acombination of various streams.

The heavy hydrocarbon flow of feedstock A includes percentages ofcatalyst damaging basic nitrogen between 200 and 3500 ppm.

The present invention includes a process for the fluid catalyticcracking of heavy hydrocarbon feedstocks in FCC Units with multiplerisers. The present process is especially targeted at FCC units thathave, at least, two risers, with different diameters, in order to beable to process two segregated feedstocks with different levels of basicnitrogen.

The FCCU useful in the process of the invention is State of the Artequipment, used, for example, in U.S. Pat. No. 4,874,503.

Feedstock A with a lower level of basic nitrogen is fed into the mainriser of the unit, in other words, in the riser with the greaterdiameter that allows a greater feedstock output. Feedstock B, which isrich in basic nitrogen, is segregated and fed, simultaneously, into asecondary riser, with a lower output.

The contact of the greater volume of the hydrocarbon feedstock in themain riser with the regenerated catalyst, free of the presence ofcompounds that neutralize the acid sites of the catalyst, favors processconversion and selectivity, since the basic nitrogen that is mostdamaging to the active sites of the catalyst, will be concentrated inthe feedstock in the other “riser(s)”.

Simultaneously, feedstock B (with its higher level of basic nitrogen),that is fed into a secondary riser, is processed together with a coolingfluid towards feedstock B.

The addition of this cooling fluid (that may or may not be inert), withthe purpose of increasing the circulation in the secondary riser, due tothe transient cooling effect in the riser, the unit gains control of thetemperature of the riser. Being thus, so that a constant temperature maybe maintained in the riser after introducing the cold stream, thecatalyst intake valve towards the riser is opened, with the consequentautomatic increase in circulation. The increase in circulation of thecatalyst favors the cooling of the regenerator and consequently mayguarantee that the circulation in the main riser will be maintained atthe normally processed levels, in other words, a catalyst/oil ratio maybe maintained within a range of between 4.5 and 8.5 in both risers.

The cooling fluid comprises between 5 and 60% in volume of the currentfeedstock B. The percentage that should be used depends on the type offluid used. In this case of using an inert substance, like water, thepercentage to be used will be lower, will not generate coke, which isone of cracking reaction products. Since the burning off of the coke isprocessed in the regenerator, the larger the amount of coke generated,the higher the temperature of the regenerator.

Generally the cooling fluid is a light hydrocarbon fraction with boilingpoint between 32 and 350° C. and with a density at 20/4° C. between 0.7and 1. These hydrocarbons are usually comprised of hydrocarbons C1 toC5. Alternatively the cooling of the regenerator may be accomplishedwith the help of water.

It should be noted that the required concentration range of the coolingfluid in the secondary riser may seem to be too wide, but it reflectsprecisely the great difference that exists when the use is allowed offluids whose intrinsic properties are as diverse as water and lighthydrocarbon fractions. In other words, if the processing is performedwith water, that does not react, in process conditions, barely a smallproportion of same, between 5 and 10%, in volume, is capable ofwithdrawing the heat that must be taken out of the riser to not alterthe thermal balance of the FCCU.

The same cannot be said when naphtha is used for example, which requiresthe addition of up to 60% in volume, in order to sufficiently cool thefeedstock, requiring more volume for the same cooling delta and stillsome of the generated heat must be deducted that results from possibleexothermic reactions that always occur in process conditions.

In each riser, hydrocarbons are cracked, leading to the deposit of theby-product coke on the catalyst that loses part of its activity. Afterthe risers, a stream of cracked hydrocarbons is separated from thecatalyst. The cracked hydrocarbons constitute the product of thereaction and are sent towards the fractioning systems.

The spent catalyst is sent to a rectifier vessel to recover any productsof the reaction that would otherwise be dragged towards the regeneratortogether with the catalyst. Right away, the catalyst is fed to theregenerator, where the coke deposits on the catalyst particles areburned off, with the objective of recovering the activity of thecatalyst and to produce catalyst particles regenerated at hightemperature, the heat of which is to a large extent consumed in theriser to fulfill the thermal demand for heating and vaporizing thefeedstock and for the catalytic cracking reactions, which arepredominantly endothermic.

The catalyst used for cracking a hydrocarbon feedstock may include anyof the known catalysts that are used in FCC technology. The preferredcatalysts are zeolites because of their intrinsically high activity andfor their resistance to the deactivation effects of vapor exposure tohigh temperature and metals. Normally, zeolites are dispersed in aninorganic porous carrier such as silica, alumina or zirconium. The levelof zeolite in the catalyst may reach 30% or more, by weight.

Although the present process may be used for feedstocks with differentpercentages of carbon residue, asphaltenes and metals, it is especiallytargeted at hydrocarbon feedstocks that have different levels of basicnitrogen, in other words, with at least 200 ppm of difference betweenthe feedstock.

Feedstocks that may be feasibly processed using the present process aredirect distillation heavy gas oil, vacuum gas oil and coker gas oil,deasphalted oils, atmospheric residues and vacuum residues, used aloneor mixed in any proportion.

Hydrocarbon feedstock A is usually made up of heavy hydrocarbon streamswith a boiling point of between 340° C. and 560° C. and an ° API ofbetween 8 and 28.

A typical feedstock A for cracking as corresponds to the invention wouldbe a vacuum treated heavy gas oil with a boiling point of between 380°C. and 540° C. and an ° API of between 15 and 22.

Heavy hydrocarbon feedstock B encompasses vacuum treated heavy gas oil,direct distillation heavy gas oil, atmospheric residue, vacuum residues,deasphalted oil, alone or mixed in any proportion.

Hydrocarbon feedstock B with a level of catalyst damaging basic nitrogenof between 1000 and 3500 ppm of b) is generally a heavy hydrocarbonstream with a boiling point of between 340° C. e 560° C. and an ° APIbetween 8 and 28.

A typical heavy hydrocarbon feedstock B would be a deasphalted oil, withan initial boiling point of between 320 and 390° C. and an ° API ofbetween 12 and 18.

The hydrocarbon feedstocks A of a) and B of b) are introduced into theeach of the risers (7) and (8) at temperatures between 100 and 450° C. Atypical temperature for introducing feedstocks is between 240 and 360°C.

So, according to the invention, feedstock B (with a higher level ofcatalyst damaging basic nitrogen) is routed towards a secondary riser(8), with a lower output, allowing the catalyst, in the main riser (7),with a higher output, to remain with a higher catalytic activity.

To avoid generating excessive coke in the secondary riser (8), whichwould compromise the catalyst/oil ratio in the main riser (7), a coolingfluid or other substance that will remove heat must be added in thesecondary riser (8). Said fluid may be a light hydrocarbon fraction.

Following are described the steps of the process using as a base, theschematic drawing, FIG. 1 attached, that does not intend to limit thescope of the present invention, but rather to illustrate a possibleconfiguration of the process based on the claims made for the presentinvention.

The FCC process of the present invention consists of a reactor (1), aregenerator (3), two elongated reaction zones or risers, (7) one mainriser and (8) one secondary riser, that provide two zones forconversion. The circulation and contact of the catalyst with thefeedstock proceeds as described below.

Therefore, two pipes (4 and 5) extend from out of the regenerator (3)through which the regenerated hot catalyst passes towards the conversionzones. The catalyst passes through duct (5) through the lower portion ofthe pipe (6) on the main riser, that is duct (7). The catalyst passesthrough duct (4) towards the lower pipe (9) of the secondary riser, thatis duct (8). Fluidizing drag gas “gas lift”, normally employed in FCCUnits, with the purpose of accelerating the catalyst, is introducedthrough ducts (11), with one duct for each riser, entering into contactwith the catalyst, in the lower pipes of the risers, maintaining thecatalyst in a fluidized state. The distribution of the fluidizing draggas in the lower pipes of the riser is performed preferentially by aperforated ring or even by a perforated plate. These typicaldistribution devices are familiar to experts in the field.

The regenerated hot catalyst will normally be at a temperature within arange of between 650 and 760° C. with a typical range of between 680 upto 732° C. as it leaves the ducts (4 and 5).

The residence time of the catalyst particles in the risers (7) and (8)varies between 1.3 and 8 seconds, preferably between 1 and 5 seconds.

Each riser (7) and (8) provides a conversion zone for cracking of thehydrocarbon feedstock. The conversion zone includes a vertical duct forpneumatic transport of the regenerated hot catalyst mixture coming fromthe regenerator with the feedstock. The feedstock is introduced intoeach riser (7) and (8), through injectors (12 and 13). Before contactwith the catalyst, the feedstock presents a temperature of between 100and 450° C., preferably between 240 and 360° C.

Reaction temperature is controlled in the upper part of each of therisers (7) and (8), usually within a range of 510 to 570° C., preferablybetween 520 and 560° C. This control is made through a conventionaltemperature measurement device, together with a controller and a signaltransmission device that acts upon a control valve.

The present invention establishes that, given a different unit thatprocesses a mixture of different feedstocks, such as, for example,mixtures of vacuum gas oil and coker gas oil, the feedstock richest incatalyst damaging basic nitrogen, in this case, coker gas oil, should besegregated and processed in riser (8), preferably with the lowerdiameter, in mixtures that contain varying amounts of coker gas oil(between 95 and 40% in volume), and of a cooling fluid (between 5 and60% in volume) to remove heat in the referenced riser.

Simultaneously, the vacuum gas oil, in this case, the feedstock thatmust be poorer in basic nitrogen (by at least 200 ppm), should be fedinto the main riser (7), preferably with the greater diameter, where itwill be possible to maximize the conversion, as a function of avoidingthe neutralization of the acid sites by the highly reactive basicnitrogen present in the coker gas oil.

It should be emphasized that the difference of 200 ppm less of basicnitrogen stipulated for the poorer feedstock destined for the main riseris only a reference limit that becomes attractive when feedstocks aresegregated to be submitted to the new process. However, differenceshigher than 200 ppm, such as, for example, 500, 1000, 1500 ppm, or more,are preferable because they optimize the conversion and the selectivityof the process even more.

When dealing with feedstocks A and B made up of mixtures of differentstreams, if the present invention were to be applied, feeding mixture Bis segregated from the high level of catalyst damaging basic nitrogenstreams, in the secondary riser (8), together with a light stream thatis responsible for removing heat from the secondary riser and feedingmixture A (which is a less contaminated stream) into the main riser (7),so as to maintain the level of basic nitrogen in segregated streams B,fed into secondary riser (8), with up to 3500 ppm more than the level ofbasic nitrogen present in streams A fed into the main riser (7).

The reacted mixture made up of the spent catalyst and the hydrocarbonvapors produced by the reaction are then discharged from the end of theriser, passing through the catalyst separation device, located insidethe reactor and not shown in FIG. 1 since it is already very well knownby technologists in the field. The separation device is normally acyclone type, but any arrangement of the separators may be used toremove spent catalyst from the product stream. Hydrocarbons flow offtowards duct (10), then is sent to the fractioning sections and to therecovery of the traditional products of catalytic cracking units, whilethe catalyst particles covered with coke (spent catalyst), flow towardsrectifier (2), where vapor, running against the stream, removes theabsorbed hydrocarbons on the surface of the catalyzer.

The rectified spent catalyst passes to regenerator (3), forming afluidized bed, where coke is typically burned off of the surface of itsparticles by coming into contact with an oxygenized gas (usually air),that enters into regenerator (3) through an entrance in the bottom ofthe regenerator. Cyclone type separators, installed on the inside of theregenerator (due to its simplicity, it is not shown in FIG. 1), removecatalyst particles dragged by the combustion gas, returning them to thecatalyst bed before the exit of the gas. Combustion of the coke catalystparticles heat the catalyst and the combustion gases.

The present invention will now be illustrated by the following examplethat should not be considered as a limitation of same.

EXAMPLE 1

In a typical example, obtained through testing on an FCC pilot unit,simulating an operation with two risers, with a feedstock output of 720g/h and the main data of operation of which are found in TABLE 1 below,the results are compared of a catalytic cracking of a feedstock made upof 70% vacuum gas oil and 30% deasphalted oil, that in the State of theArt system is cracked as a mixture, with the procedure performedaccording the present invention, where the feedstock is cracked withsegregation of the feedstocks, in separate risers. TABLE 1 70% HGO + 30%DESO HGO e DESO (v/v) MIXTURE - SEGREGATED- DATA Case A Case BTemperature of the 530 530 reaction, ° C. C/O Ratio riser 1 - main 7.38.16 riser 2 - secondary 7.3− 4.50 Temperature of feedstock, ° C. riser1 - main 225 220 riser 2 - secondary 225 290 Yield, % by weightCombustible gas 6.8 5.4 LPG 15.6 16.1 Gasoline 37.9 39.0 Light cycle oil(LCO) 15.7 17.6 Decanted oil 17.6 15.0 Coke 6.4 6.9

In the column corresponding to Case A in TABLE 1 illustrative of theState of the Art, feedstock mixture is presented as a unit feedstock fedinto two risers under the operational conditions described in the table.The temperature of the reaction is 530° C. In comparison, we have CaseB, which is the proposal of the invention to crack segregated feedstocksA and B, in separate risers, simultaneously, maintaining the samereaction temperature in each riser, but with different feedstocktemperatures.

As a consequence of the application of the process of the invention anincrease is observed in the yield of LPG and gasoline and a reduction inthe production of decanted oil and combustible gas, in this waymaximizing the production of noble derivatives. The differences in yieldbetween the Base Case (Case a) and an option of the present invention(Case B) are summarized in TABLE 2 below. TABLE 2 Differences in yieldfrom Case B Yield, % by weight in relation to Case B, points of % LPG +Gasoline +1.7 Combustible gas −1.4 LCO +1.9 Decanted oil −2.6

The principal characteristics of the feedstocks used in the Example areoutlined in TABLE 3.

In the Example of the invention, beyond the profits from gasoline andLPG, a reduction in combustible gas is obtained and an increase inbottom conversion, for the reduction of decanted oil. The example showsthat the alternative of freeing the acid sites of the catalyst fed intothe main riser promotes greater selectivity in cracking of the mainfeedstock.

The profits of selectivity are possible with the process of theinvention since stream B (richest in catalyst damaging basic nitrogen),upon being placed into the secondary riser (8), with a lower output,allows the catalyst to be preserved in the main riser (7), with a higheroutput, thus making an increase in conversion and greater selectivitypossible.

It must be made very clear that according to the principles of theinvention several refinements and improvements can be made, all withinthe spirit of the invention, in the sense of obtaining even greaterprofits than those explained in the descriptive report, suchmodifications include changing the reaction temperatures in each riser(7), (8) as well as cooling the base of the secondary riser (8) with afluid such as water, to refine the removal of heat and increase thecirculation. TABLE 3 Feedstocks Mixture (% v/v) HGO (70) + PropertiesHGO DESO (30) DESO Density, °API 17.8 17.8 16.3 Density at 20/4° C.0.9442 0.9443 0.9535 Aniline Point, ° C. 90.2 92.6 109.8 DistillationASTM D-1160. ° C. PIE 397.3 397.2 359.0 5% vol. 412.3 416.9 477.8 10%vol. 429.5 433.1 517.6 20% vol. 451.2 450.4 555.5 50% vol. 485.5 489.8 —80% vol. 52.4 534.3 — Total nitrogen, ppm 3100 3200 3400 Basic nitrogen,ppm 1099 1172 1369 Ramsbottom carbon residue, % 1.03 1.13 4.00 weightPentane insoluble, % weight 0.1 0.1 <0.05 Sulphur, % weight 0.66 0.720.79 Polyaromatic, % weight 8.05 7.96 8.00 Kinematic viscosity, cSt AT60° C. 177.7 203.6 — At 82° C. — — 448.5 At 90° C. — — — At 100° C. 26.829.3 170.5 Carbon 13. RMN (1) Carbon, % 19.196 — 17.30 Saturated carbon,% 80.804 — 82.70 Carbon - hydrogen, % 8.023 — 6.786 Carbon - methyl, %1.121 — 0.840 Carbon - alkyl, % 6.050 — 5.316 Carbon, B, % 4.002 — 4.358Carbon/hydrogen 0.552 — 0.547 Linear n-alkanes. % 15.783 — 23.059Aromatic n-alkanes, % 11.012 — Metals, ppm Nickel <1.0 <1.0 <1.0Vanadium 1.6 1.8 4.0 Sodium 1.4 2.0 2.8Note:(1) Nuclear magnetic resonance.

The considerations and the results of Example 1 above prove that theprocess of the invention provides an optimization in cracking conversionselectivity rates when the stream that is richest in catalyst damagingbasic nitrogen is segregated to be injected into a riser with a loweroutput (8), because it leaves the main riser (7) catalyst active tofunction for more time in one feedstock with a higher output and with alower amount of basic nitrogen.

1. A Process for fluid catalytic cracking of hydrocarbon feedstocks withhigh levels of basic nitrogen, in multiple riser FCCUs, operating withfeedstocks A and B, wherein the process comprises the following steps:a) place in contact with a zeolite catalyst, in the main riser (7) ofthe FCCU, a hydrocarbon feedstock A which possesses a level of basicnitrogen at least 200 ppm lower than feedstock B that is being processedin a secondary riser (8) of the same FCCU; b) simultaneously, place incontact with the same zeolite catalyst of a), in said secondary riser(8) of the FCCU, a hydrocarbon feedstock (B) comprised of a mixture madeup of between 95 and 40%, in volume, of hydrocarbon streams with acontent of catalyst_damaging basic nitrogen of between 1000 and 3500ppm, and between 5 and 60%, by weight, of a cooling fluid capable ofincreasing the circulation in this secondary riser and of cooling theregenerator, in order to adjust the thermal balance of the FCCU andmaintain the circulation of the catalyst in the main riser, at properlevels, so that the catalyst/oil ratio remains in the range of between4.5 and 8.5; c) maintain the operation of the FCCU within the conditionsof catalytic cracking; d) recover from tube (10), products of thecracking reaction with an increase in conversion of bottom fractions, agreater proportion of gasoline and LPG, at the same time with a lowerproportion of coke and combustible gas.
 2. A Process according to claim1, wherein the FCCU includes two risers, a main riser and a secondaryriser.
 3. A Process according to claim 1, wherein the FCCU that includesthree risers, a main and two secondary risers.
 4. A Process according toclaim 1, wherein said hydrocarbon feedstock A of a) is made up of heavyhydrocarbon streams with a boiling point of between 340° C. and 560° C.and an ° API of between 8 and
 28. 5. Process according to claim 4,wherein the heavy hydrocarbon flow streams of feedstock A comprisevacuum treated heavy gas oil, direct distillation heavy gas oil,atmospheric residue, vacuum residues, deasphalted oil, alone or mixed inany proportion.
 6. Process according to claim 5, wherein one of theheavy hydrocarbon streams of A is a vacuum_treated heavy gas oil with aboiling point of between 380° C. and 540° C. and an ° API of between 15and
 22. 7. Process according to claim 4, wherein the heavy hydrocarbonflow of A comprises isolated streams and mixtures between streams thathave levels of catalyst damaging basic nitrogen of between 200 and 3500ppm.
 8. Process according to claim 1, wherein hydrocarbon feedstock Apossesses a level of basic nitrogen of at least 500 ppm lower thanfeedstock B that is being processed in the secondary riser (8) of thesame FCCU.
 9. Process according to claim 1, wherein hydrocarbonfeedstock A possesses a level of basic nitrogen at least 1000 ppm lowerthan feed stock B that is being processed in the secondary riser (8) ofthe same FCOU.
 10. Process according to claim 1, wherein hydrocarbonfeedstock A possesses a level of basic nitrogen 3500 ppm lower thanfeedstock B that is being processed in the secondary riser (8) of thesame FCCU.
 11. Process according to claim 1, wherein the catalyst is aconventional zeolite type for FCC processes of heavy feedstockscontaining basic nitrogen, with around 30% zeolite dispersed in aninorganic porous carrier.
 12. Process according to claim 1, whereinfeedstock B of hydrocarbons with levels of catalyst_damaging basicnitrogen of between 1000 and 3500 ppm of b) is a heavy hydrocarbonstream with a boiling point of between 340° C. and 560° C. and an ° APIof between 8 and
 28. 13. Process according to claim 12, wherein saidheavy hydrocarbon stream of feedstock B includes vacuum treated heavygas oil, direct distillation heavy gas oil, atmospheric residue, vacuumresidues, deasphalted oil, alone or mixed in any proportion.
 14. Processaccording to claim 13, wherein one of said heavy hydrocarbon stream offeedstock B to be a deasphalted oil, with an initial boiling point ofbetween 320 and 390° C. and an ° API of between 12 and
 18. 15. Processaccording to claim 1, wherein the cooling fluid in the secondary riser(8) of b) is a light hydrocarbon stream with boiling point between 32and 350° C. and with a density at 20/4° C. of between 0.7 and
 1. 16.Process according to claim 15, wherein a light hydrocarbon stream isadded in proportion of between 5 and 60% by volume of the total streamB.
 17. Process according to claim 1, wherein the cooling fluid of thesecondary riser (8) of b) is an inert stream.
 18. Process according toclaim 17, wherein said inert stream is water in proportion of between 5and 10% by volume of the total stream B.
 19. Process according to claim1, wherein the hydrocarbon feedstocks A of a) and B of b) are introducedinto the risers (7) and (8) at temperatures between 100 and 450° C. 20.Process according to claim 19, wherein hydrocarbon feedstocks A of a)and B of b) are introduced into the risers (7) and (8) at temperaturesof between 240 and 360° C.
 21. Process according to claim 1, whereinreaction temperatures in the risers (7) and (8) are controlled atbetween 510 and 570° C.
 22. Process according to claim 21, whereinreaction temperatures in the risers (7) and (8) are controlled atbetween 520 and 560° C.
 23. Process according to claim 1, wherein theregenerated hot catalyst which leaves the regenerator to enter into therisers (7) and (8) to be at temperatures of between 650 and 750° C. 24.Process according to claim 23, wherein the regenerated hot catalystwhich leaves the regenerator to enter into the risers (7) and (8) to beat temperatures of between 680 and 732° C.
 25. Process according toclaim 1, wherein the residence time of the catalyst particles, in therisers (7) and (8) fluctuates between 0.3 and 8 seconds.
 26. Processaccording to claim 24, wherein the residence time of the catalystparticles, in the risers (7) and (8) fluctuates between one and fiveseconds.